Lowering the resolution is not likely to make that much of a difference. If it is really taking hours then your Internet connection can not support ATV. Lowering the resolution my take the download from say two and half hours to say two hours, but that is really not much of an improvement. Generally even with a moderate Internet connection a movie will start playing with in 30 - 90 seconds.
When you say from iTunes are you talking about locally from your computer, or from iCloud? If it is locally from your computer you have a local network issue as it should start pretty close to instantly.